1. Field of the Invention
This invention relates to a device for directing signals to an antenna, and more particularly, to a fiberoptic device for directing test and calibration signals to the individual transmit-receive modules of a phased array antenna.
2. Background Information
The utilization of traditional far-field techniques for testing and calibrating the individual transmitreceive (T/R) modules of a phased array antenna requires a small number of RF field strength measurements to be taken at such a distance (range) from the modules that the modules appear as a point source of radiation. Due to the large size of many commercially available phased array antennas, the range required to implement far-field testing techniques is well over a mile in length. Over these great distances, it is extremely difficult to control environmental factors such as weather, reflections from nearby objects and external sources of interfering signals. It has been found that the inability to control these factors makes far-field testing of large phased-array antennas extremely impractical.
Because of the difficulties encountered with traditional far-field testing, it is preferred to utilize a near-field testing scheme to test and calibrate large-scale phased array antennas.
Near-field testing, however, requires the construction of special facilities and equipment to both house the antenna and perform the alignment, testing and calibration of the individual T/R modules. This technique involves the taking of a large number of measurements at a very close proximity to the face of the antenna array. An RF probe mounted on a large X-Y scanner or plotter is used to make these measurements, and known digital signal techniques are utilized to convert the measurements to the same types of antenna patterns which would be obtainable by far-field testing.
Although this method avoids many of the environmental difficulties encountered with far-field testing, known near-field test devices utilize a coaxial cable to transmit the test and calibration signals from the signal generating equipment to the RF probe. The signals are thereafter introduced into a waveguide or feedhorn, which is also mounted on the scanner, and the open end of the waveguide is passed in proximity to the plurality of individual T/R modules secured to the antenna face. Since the RF probe and waveguide must be moved both horizontally and vertically by the X-Y scanner to provide test and calibration signals to the T/R modules in succession, the coaxial cable must have a length sufficient to freely travel with the RF probe and waveguide. The length of the coaxial cable required to permit free travel of the RF probe and waveguide causes the cable to be extremely heavy and requires a substantial coaxial cable support system which is separated from the X-Y scanner. In addition, an end portion of the coaxial cable must be secured to the structural framework of the scanner in order to permit a tension-free connection between the cable and RF probe. As a result, the scanner structural framework must be built to withstand the combined weight of the RF probe, waveguide, and a portion of the coaxial cable.
The massive scanner structural framework must be properly shielded with suitable microwave absorbing material, and the additional weight of the microwave shielding necessitates a still further enlargement of the framework of the X-Y scanner. It can be appreciated that fabricating an X-Y scanner having a framework sufficient to totally support the RF probe, waveguide, cable and shielding material is extremely costly. In addition, the sheer weight of the scanner framework makes it difficult to move the scanner the incremental distances required to align the open end of the waveguide with the individual T/R modules in the array.
Therefore, there is a need for an improved system for directing test and calibration signals to the individual T/R modules of a phased array antenna in a near-field test facility which overcomes the deficiencies of presently used systems. The improved system must include means for passing signals from the signal generating equipment to the RF probe which eliminates the need for coaxial cable and its attendant cable support system. Eliminating the use of coaxial cable and its support system greatly reduces the loading experienced by the X-Y scanner, and allows the scanner framework to be reduced to both size and weight.